This invention relates to a pane, notably a pane used in the aeronautical sector.
The pane may be monolithic or laminated, of glass or of plastics material, or it may also be a composite pane composed of an assembly of rigid sheets, of glass and/or of plastics material, and of flexible sheets.
A composite pane used in the aeronautical sector is, commonly, composed of at least two sheets of thermally or chemically toughened glass, between which there is an intermediate sheet of plastics material which may, if desired, be composed of several plies or sheets.
When the pane is in the usual conditions of a high altitude flight it is subjected, on the one hand, to considerable differences of pressure between its two faces and, on the other hand, to also considerable differences of temperature between the median part and the peripheral part of the pane. Furthermore, it is also subjected to vibrational loadings. As indications, the pressure difference may reach 900 millibars and the temperature difference may reach approximately 70.degree. C. To these already high stresses there can be added stresses due, for example, to an impact in flight, in particular at low temperature, such as the strike of a bird, or other local stresses.
As a result of these particularly high loadings, such panes deteriorate with use after a greater or lesser period.
Solutions have already been proposed for reducing the risks of failure of the pane. One of the most common causes of failure is the difference in the coefficients of thermal expansion of the glass and of the intermediate sheet of plastics material which is, generally, of polyvinyl butyral. One solution consists of introducing a material having a low coefficient of thermal expansion into the marginal part of the pane, between the intermediate sheet and the glass. Another solution consists of placing a continuous film of a material of a difference nature having a lower modulus of elasticity between the intermediate sheet and the glass. These solutions may, however, lead to optical defects in the pane. There has also been proposed, in the document EP 508 864, an intermediate sheet of polyvinyl butyral composed of several plies, the plasticizer content of the plies in contact with at least one sheet of glass being higher than in the internal plies. This characteristic makes possible, also, a reduction and indeed an elimination of the known stresses leading to a cleavage of the glass, that is to say a rupture in a plane substantially parallel to the plane of the glass, notably when it is chemically toughened.
These panes of the earlier documents have exhibited, as a whole, improved properties. They have satisfactory working lives. Nevertheless, in certain cases, important precautions are necessary during the production of the pane.
The present invention overcomes these disadvantages.